9
SampleVial Heater andMixer
Once the sample is placed inside a clean, non-contaminating vial and the vial is
sealed, volatile compounds from the samplewill partition into the headspace until a
state of equilibrium is reached. The rate at which volatile compounds partition out of
the samplematrix and into the headspace, aswell as the equilibrium concentration
of volatile compounds in the headspace depends on several parameters (see also
Introduction
of this guide).
Temperature, time, andmixing can be
used to improve the transfer of volatile
analytes from the sample into the
headspace of the vial.Adjusting the
temperature of the samplewill change
the solubility of the analyte in the
samplematrix and can be used to drive
the equilibrium in favor of the gas
phase. Sufficient timemust be built into
the sample cycle in order to achieve a
constant state of equilibrium. Some samplematrices require longer equilibration
times due to physical characteristics like high viscosity. Shaking or vibrating the vial
during heating can assist in achieving equilibriummore quickly by exposingmore
sample surface area for the transfer of volatile analytes to the headspace.
Sampling
There are several techniques used to transfer samples from the vial to the
GC.When using a
gas-tight syringe
for sampling, heat the syringe to a
temperature comparable to the sample vial temperature. Thisminimizes
pressure differences and condensation problems. To prevent carryover from
inside the syringe, flush the syringe after each injection. Because gas-tight
syringe samplers inject through theGC injection port septum, ensure the
septum iswellmaintained to decrease the possibility of a leak.
For
balanced-pressure sampling
instruments, analysts should consider the
inertness and efficiency of the components thatmake up the sample pathway
inside the autosampler. If sensitive compounds are being analyzed, an inert
pathway should be used to decrease possible adsorption.Materials such as
stainless steel, nickel, Silcosteel
®
andTeflon
®
coatings, orKEL-F
®
parts can
be used tominimize sample adsorption and peak tailing. Transfer line internal
diameter should be as narrow as possible to helpmaintain narrow sample band
widths and symmetrical peak shapes (see the following optimization of transfer lines
formore information).Analysts also should ensure that balanced-pressure instru-
ments are leak-free and operatewith the least amount of dead volume in the sample
flow path. Thiswill help obtain optimal peak shape and sensitivity.
When using
pressure-loop sampling
instruments, the same concerns apply aswith
gas-tight syringe and balanced-pressure systems. Inert sample pathways and low
dead volume systemswill yield the best chromatographic performance. In pressure-
loop systems, a gas sampling valvewith a sample loop is used to transfer the sample
from the headspace unit to theGC.Adequate purging of the sample valve and loop
will guard against sample carryover. If low response or broad peaks are observed, it
may be necessary to increase the sample vial pressure to ensure that the sample loop
is being completely filledwith headspace sample. If there are extraneous peaks
present due to carryover ofmatrix contaminants, increase the sample valve tempera-
ture to prevent sample carryover, condensation, and contamination.
✶
Shaking or vibrating the
vial during heating can
assist in achieving
equilibrium.
✶
For technical support, call
800-356-1688, ext. 4
(814-353-1300, ext. 4)
or call your local
Restek representative.
